Exponential conversion circuit and variable gain circuit

ABSTRACT

In a master block, the exponential conversion characteristic is determined on the basis of a common mode reference voltage and a reference voltage. In a slave block, the exponential conversion characteristic determined with the master block is used to create a control voltage and a gain control signal on the basis of a common mode reference voltage and a reference voltage. For example, a gain of the variable gain amplifier is controlled by using this gain control signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2000-280829, filed Sep. 14,2000, the entire contents of which are incorporated herein by reference,

BACKGROUND

The present invention relates to an exponential conversion circuithaving a function of exponentially changing a gain of a variable gainamplifier on the basis of a gain control signal.

In recent years, mobile communication devices represented by a portablephone or the like are briskly developed It is important that thesemobile communication devices are small in size and light in weightbecause these devices are based on a presupposition that the devices canbe carried by a user.

Consequently, at present, it rarely happens that these mobilecommunication devices comprise a combination of a plurality ofindividual components (functions). The mobile communication devicescomprise ASIC's mixedly providing the plurality of functions. As aconsequence, size reduction and weight reduction of the mobilecommunication devices are realized.

By the way, such mobile communication devices naturally have a sendingand receiving circuit for sending and receiving electronic waves for theexchange of information by wire (electric waves). In the IF(intermediate frequency) portion of the sending and receiving circuit, avariable gain amplifier is arranged, and this variable gain amplifierhas a function of adjusting an IF signal to an appropriate level.

For example, there is available a code division multiple access (CDMA)method as one of the mobile communication methods. In the CDMA method,the control of the sent electric power in a mobile station becomesindispensable, a wide scope gain control of 70 dB or more is demandedfor the variable gain amplifier used in the IF portion.

Generally, in order to perform such wide scope gain control with thevariable gain amplifier, it is necessary to exponentially adjust asignal level with respect to the gain control signal, Furthermore, inorder to facilitate the gain control, it becomes important that therelation between the control input signal and the decibel display outputsignal has a linear configuration over a wide scope.

Furthermore, portable phones are based on a presupposition that thephones are carried by a user. Consequently, it is desired that the gainof the variable gain amplifier used therein has a small dependency on atemperature change resulting from a change in the environment in whichthe phones are used. Furthermore, a gain error must be suppressed whichis caused by a disparity in a threshold value of a MOS transistorresulting from a manufacturing process of an integrated circuit.

However, for example, the constant maintenance of the characteristic forexponentially changing the gain of the variable gain amplifier withrespect to the gain control signal, and the change of the decibeldisplay output signal with respect to the control input signal becomevery difficult for the following reasons.

In the beginning, the variable gain amplifier will be explained.

As shown in FIG. 1, a variable gain amplifier and a gain control circuitthereof comprise a MOS transistor (CMOS circuit).

Here, the MOS transistor is generally used in the double characteristicarea (strong inversion area) but can be used in a sub threshold area(weak inversion area). In this case, an exponential operation isconducted and the transmission characteristic can be approximatelydescribed in the following manner. $\begin{matrix}{I_{D} = {{kx}\frac{W}{L}\exp \frac{V_{GS}}{{nV}_{T}}}} & (1)\end{matrix}$

Incidentally, in the expression (1), symbol I_(p) denotes a draincurrent of a MOS transistor, symbol W denotes a channel width of the MOStransistor, symbol L denotes a channel length of the MOS transistor,symbol V_(GS) denotes a voltage between the gate and the source of theMOS transistor, and symbol V_(T) denotes a thermal voltage. Symbol n isa constant. Furthermore, symbol K_(x) has a value associated with aconductance of the MOS transistor. Symbol Kx depends on themanufacturing process of the integrated circuit together with theconstant n.

By the way, in FIG. 1, a variable gain amplifier 702 can vary the gainwith a bias current Ibias. Furthermore, the bias current Ibias becomesequal to a drain current I_(D) of a MOS transistor M701 with the currentmirror circuits M702 and M703 inside of a gain control circuit 701.

On the other, when the MOS transistor M701 in the variable gain controlcircuit 701 is allowed to be operated in a weak inversion area to give again control signal Vc to a gate of the MOS transistor M701, the draincurrent I_(D) of the MOS transistor M701 changes exponentially with thechange in the gain control circuit Vc.

That is, as a consequence, the gain of the variable gain amplifier 702changes exponentially with the change of the gain control signal Vc.

However, the following problem is generated in order to directly use thecharacteristic of the expression (1) in the circuit of FIG. 1.

That is, when logarithm on both sides of the equation (1) is taken, thefollowing equation is provided. $\begin{matrix}{{\log \quad I_{D}} = {{\log \quad {kx}} + {\log \quad \frac{W}{L}} + {\left( \frac{1}{{nV}_{T}} \right)V_{GS}}}} & (2)\end{matrix}$

Here, as described above, in expression (2), symbol K_(x) is affected bythe influence of the manufacturing process of the integrated circuit,the transmission characteristic (expression (2)) of the MOS transistor,namely, the exponential conversion characteristic changes with themanufacturing process, specifically, a disparity in the thickness andthe processing generated at the time of the manufacturing process.

Furthermore, a third item on the right side of the expression (2)determines the exponential conversion characteristic (characteristic ofthe exponential correlation). However, with respect to a heat voltageV_(T), in order to maintain the temperature dependency, the exponentialconversion characteristic also changes depending on the temperaturechange in the case where a temperature change is generated in the MOStransistors M 701, M 702 and M 703 in the gain control circuit. As aconsequence, the variable scope (gain characteristic) of the gain of thevariable gain amplifier 702 changes.

Incidentally, in the gain control circuit 701 of FIG. 1, even when abipolar transistor is used instead of the MOS transistor 701, theexponential conversion characteristic (characteristic of the exponentialcorrelation) comes to have a temperature dependency for the same reasonsas described above.

Consequently, in the case where the exponential conversioncharacteristic of an active device is directly used in the variable gainamplifier 702, an error is generated in the exponential conversioncharacteristic resulting from a change in the environment (temperaturechange) and a manufacturing process of the integrated circuit so that adesired exponential conversion characteristic cannot be obtained.

Furthermore, with respect to the variable gain amplifier 702 used in awireless receiver, it is important to linearly change a decibel displayoutput signal with respect to the control input signal. Furthermore, itis required that the temperature dependency of the exponentialconversion circuit is small, and the exponential conversioncharacteristic is not affected by the characteristic change of theactive device resulting from the manufacturing process of the integratedcircuit.

In this manner, in the case where exponential operation (characteristic)of the active device (MOS transistor) is directly used in the control ofthe variable gain amplifier 702, the variable gain amplifier 702 cannotmaintain at a constant level a variable scope (gain characteristic) ofthe gain of the variable gain amplifier 702 with respect to the variablescope of the gain control signal Vc because of the change of thetemperature environment of the active device, the characteristic changeof the active device due to the manufacturing process of the integratedcircuit, or the like.

From this, it is demanded that the gain is exponentially changed withrespect to the gain control signal, that is, the relation between thechange in the gain control signal and change in the gain which is adecibel display is linear from the viewpoint of facilitation of thecontrol in the case where the gain is controlled in the wirelesscommunication device. Furthermore, in such a case, it is demanded thatthe change ratio of the gain characteristic with respect to thetemperature is definite, and the gain characteristic does not changewith the characteristic change of the active device resulting from themanufacturing process of the integrated circuit with the result that therealization of the gain control circuit for realizing these demands isdesired.

As described above, in the conventional variable gain amplifier and thegain control circuit for controlling the gain, there is a problem thatthe variable scope (gain characteristic) of the gain in the variablegain amplifier with respect to the variable scope of the gain controlsignal cannot be maintained on a definite level because the gaincharacteristic of the variable gain amplifier changes resulting from thecharacteristic change of the active device and the temperature change.

SUMMARY

An exponential conversion circuit according to the aspect of the presentinvention comprises:

a first voltage conversion circuit for converting first and secondreference voltages to first and second differential output voltages;

a first exponential conversion device for creating a first outputcurrent which changes exponentially with respect to the firstdifferential output voltage;

a second exponential conversion device for creating a second outputcurrent which changes exponentially with respect to the seconddifferential output voltage;

a current comparison circuit for changing the first gain control signalin accordance with a ratio of the first and the second output currents;

a second voltage conversion circuit for converting a control inputvoltage and a first reference input voltage into third and fourthdifferential output voltages, respectively, on the basis of a first gaincontrol signal; and

a third conversion device for creating a third output current whichchanges exponentially with respect to the third and the fourthdifferential output voltages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing one embodiment of a conventional exponentialconversion circuit and a variable gain amplifier.

FIG. 2 is a diagram showing an exponential conversion circuit accordingto a first embodiment of the present invention.

FIG. 3 is a diagram showing an exponential conversion circuit accordingto a second embodiment of the present invention.

FIG. 4 is a diagram showing an exponential conversion circuit accordingto a third embodiment of the present invention.

FIG. 5 is a diagram showing an exponential conversion circuit accordingto a fourth embodiment of the present invention.

FIG. 6 is a diagram showing an exponential conversion circuit accordingto a fifth embodiment of the present invention.

FIG. 7 is a diagram showing a concrete example of a voltage conversioncircuit.

FIG. 8 is a diagram showing a concrete example or a current comparisoncircuit.

FIG. 9 is a diagram showing a certain exponential conversion circuitaccording to a sixth embodiment of the present invention.

FIG. 10 is a diagram showing a certain exponential conversion circuitaccording to a seventh embodiment of the present invention.

FIG. 11 is a diagram showing a certain exponential conversion circuitaccording to an eighth embodiment of the present invention.

FIG. 12 is a diagram showing one example of an operation of anexponential conversion circuit according to the present invention.

FIG. 13 is a diagram showing a first embodiment of a system comprisingan exponential conversion circuit and a variable gain amplifier.

FIG. 14 is a diagram showing a concrete example of a variable gainamplifier.

FIG. 15 is a diagram showing a second embodiment of a system comprisingan exponential conversion circuit and a variable gain conversioncircuit.

FIG. 16 is a diagram showing a third embodiment of a system comprisingan exponential conversion circuit and a variable gain conversioncircuit.

DETAILED DESCRIPTION

Hereinafter, referring to the drawings, there will be explained indetail an exponential conversion circuit and a variable gain circuitusing the exponential conversion circuit according to the presentinvention.

It is assumed that in the exponential conversion circuit and thevariable gain circuit of the present invention, a voltage conversioncircuit is an ideal linear circuit. However, when a voltage conversioncircuit with a variable magnification ratio is constituted by using anormal MOS transistor or a bipolar transistor, the input and outputcharacteristic shows a weak linearity. It is possible to assume that thevoltage conversion circuit conducts a linear operation with no attentionto the non-linearity. However, there as a case in which a demand on thelinearity is very strict in the system. In such a case, the correctionof the linearity is required.

Here, in the input and output characteristic of the exponentialconversion circuit, an output signal of a decibel display is denoted byY, a common-mode feedback amount is set to Y0, a ratio of a voltageconversion circuit is denoted by G, an input signal is denoted by X, anequation of Y=Y0+G·X is provided in the case where the output signal Yand the input signal X are in a linear relation. However, Y0 and G areconstants which are determined with a device value of a deviceconstituting a circuit form and a circuit.

Furthermore, the above input and output relation equation can beobtained by using a bipolar transistor and a weak inversion area of theMOS transistor. However, Y0 and G changes with the temperature, andbecomes a value different from the value at the time of measurement withthe change in the manufacturing process.

In the exponential conversion circuit, the temperature dependency of Y0and G can be made small. At the same time, the influence by themanufacturing process of Y0 and G can be alleviated. In particular, theexponential conversion circuit of the present invention makes themagnification rate G of the voltage conversion circuit variable with thecontrol signal Z. The exponential conversion circuit comprises a masterexponential conversion circuit including a feedback circuit and a slaveexponential conversion circuit for actually conducting the exponentialconversion.

In such a structure, the voltage conversion circuit can vary the gain(magnification rate). However, it is very difficult to secure a wideinput signal range while maintaining the variable scope on a wide level.As a consequence, the ratio G depends not only upon the control signal Zbut also upon the input signal X, and the ratio G changes. In this case,a relational expression Y=Y0+G(X, Z)·X can be obtained. Here, G(X, Z)means that the ratio G can be represented with the function of X and Z.

By the way, when the control signal Z is given so as to eliminate thechange of the magnification ratio G with respect to the change of theinput signal X, an exponential conversion circuit can be realized inwhich the ratio G becomes ostensibly definite. Here, suppose that Z forsetting the ratio G to a definite level is represented in a linearexpression, an expression of Z=A+B·Z can be obtained. Then, the targetexponential conversion operation (calculation of constants A and B) isconducted with two master exponential conversion circuits, a linearexpression circuit to which the input signal X is input for outputtingan output signal Z (=A+B·Z) and a slave exponential conversion circuitfor actually conducting the exponential conversion of the X.

Furthermore, suppose that Z for setting the ratio G to a definite levelis represented in a second-order linear expression, an expression ofZ=A+B·Z+CX² can be obtained. Then, an operation of the targetexponential conversion (calculation of constants A, B and C) isconducted with three master conversion circuits, and a second orderliner expression circuit to which an input signal input for outputtingan output signal Z (=A+B−X+C−X²) and a slave conversion circuit foractually conducting X exponential conversion circuit.

In this manner, in the case where it is required that Z is formed into apolynomial expression in order to set G (X, Z) to a definite level, themaster exponential conversion circuit is provided in a number largerthan the degree of the polynomial expression. As a consequence, theexponential conversion operation is realized with the polynomialconversion circuit in which the polynomial expression is determined onthe basis of the output signal of the master exponential conversioncircuit and the slave exponential conversion circuit which is controlledwith the signal converted with this polynomial circuit.

In the exponential conversion circuit of the present invention, thefirst exponential conversion circuit determines the second exponentialconversion characteristic of the second exponential conversion circuit.That is, when the control input signal is exponentially converted to asecond gain control signal on the basis of the second exponentialconversion characteristic, the second gain control signal ceases to beaffected by the characteristic change and the temperature change of theactive device or the like with respect to the control input signal.

Consequently, a second gain control signal output from the exponentialconversion circuit of the present invention is used, for example, as acontrol signal of the variable gain circuit (variable gain amplifier),the gain control which does not depend on the characteristic change andthe temperature change of the active device or the like can be realized.

FIG. 2 is a diagram showing an exponential conversion circuit accordingto a first embodiment of the present invention.

Two reference input voltages Vref1 and Vref2 are input to the voltageconversion circuit 1A. The voltage conversion circuit 1A converts thereference input voltages Vref1 and Vref2 to differential output voltagesVd1 and Vd2 on the basis of the magnification ratio determined with again control signal Vgmcont.

An exponential conversion device 2A creates an output current whichchanges exponentially with respect to the differential output voltageVd1 while the exponential conversion device 2B creates an output currentwhich changes exponentially with respect to the differential outputvoltage Vd2. These output currents are input to a current comparisoncircuit 3. The current comparison circuit 3 changes the values of thegain control signal Vgmcont in accordance with the ratio of the outputcurrent of the exponential conversion devices 2A and 2B.

Furthermore, the reference input voltage Vref1 and the control inputvoltage Vc are input to the voltage conversion circuit 1B. The voltageconversion circuit 1B converts the reference input voltage Vref1 andcontrol input voltage Vc to differential output voltages Vd3 and Vd4 onthe basis of the ratio which is determined with the gain control signalVgmcont.

An exponential conversion circuit 2C creates an output current Ioutwhich changes exponentially with respect to the differential outputvoltage Vd1. An output current Iout changes exponentially with respectto the change in a control input voltage Vc.

FIG. 3 is a diagram showing an exponential conversion circuit accordingto a second embodiment of the present invention.

The exponential conversion circuit of the present invention ischaracterized in that the voltage conversion circuits 1A and 1B have acommon-mode detection circuit (CMD) 21 and a common-mode feedbackcircuit (CMF) 20.

Two reference input voltages Vref1 and Vref2 are input to the voltageconversion circuit 1A. The voltage conversion circuit 1A has acommon-mode detection circuit 21 and the common-mode feedback circuit20. Furthermore, the logarithm conversion device 5 creates a referencevoltage Vcom1 which becomes a logarithm of the reference input currentIref having a temperature characteristic. This reference voltage Vcom1is given to the common-mode feedback circuit 20. Then, the voltageconversion circuit 1A converts the reference input voltages Vref1 andvref2 into differential output voltages Vd1 and Vd2 on the basis of theratio determined with the gain control signal Vgmcont.

The exponential conversion device 2A creates an output current whichchanges exponentially with respect to the differential output voltageVd1 while the exponential conversion device 2B creates an output currentwhich changes exponentially with respect to the differential outputvoltage Vd2. These output currents are input to the current comparisoncircuit 3. The current comparison circuit 3 changes the value of thegain control signal Vgmcont in accordance with the ratio of the outputcurrent of the exponential conversion devices 2A and 2B.

Furthermore, the reference input voltage Vref1 and the control inputvoltage Vc are input to the voltage conversion circuits 1B. The voltageconversion circuit 1B has a common-mode detection circuit 21 and acommon-mode feedback circuit 20. Furthermore, the logarithm conversioncircuit 5 creates a reference voltage Vcom1 which becomes a logarithm ofthe reference input current Iref having a temperature characteristic.This reference voltage Vcom1 is given to the common-mode feedbackcircuit 20. Then, the voltage conversion circuit 1B converts thereference input voltage Vref1 and the control input voltage Vc todifferential output voltages Vd3 and Vd4 on the basis of the ratiodetermined with the gain control signal Vgmcont.

The exponential conversion circuit 2C creates the output voltage Ioutwhich changes exponentially with respect to the differential outputvoltage Vd1 out of the two out voltages of the voltage conversioncircuits 1B in this embodiment. The output current Iout changesexponentially with respect to the change in the control input voltageVc.

Incidentally, the exponential conversion devices 2A, 2B and 2C cancomprise, for example, electric field effect transistors (MOSFET's)which are operated in a weak inversion region. Furthermore, theexponential conversion devices 2A, 2B and 2C can also comprise bipolartransistors instead of the electric field effect transistor.

FIG. 4 shows an exponential conversion circuit according to a thirdembodiment of the present invention.

Two reference input voltages Vref1 and Vref2 are input to the voltageconversion circuit 1A. The voltage conversion circuit 1A has acommon-mode detection circuit and a common-mode feedback circuit in thesame manner as FIG. 3. The voltage conversion circuit 1A converts thereference input voltages Vref1 and Vref2 to the differential outputvoltages Vd1, Vd2 on the basis of the ratio determined with the gaincontrol circuit Vgmcont1.

The exponential conversion device 2A creates the output current whichchanges exponentially with respect to the differential output voltageVd1 while the exponential conversion device 2B creates the outputcurrent which changes exponentially with the differential output voltageVd2. These output currents are input to the current comparison circuit3A. The current comparison circuit 3A changes the value of a gaincontrol signal Vgmcont1 in accordance with the ratio of the outputcurrent of the exponential conversion devices 2A and 2B.

Two reference input voltages Vref1 and Vref3 are input to the currentconversion circuit 1B. The voltage conversion circuit 1B has acommon-mode detection circuit and a common-mode feedback circuit in thesame manner as FIG. 3. The voltage conversion circuit 1B converts thereference input voltages Vref1 and Vref2 to differential output voltagesVd3 and Vd4 on the basis of the ratio determined with a gain controlsignal Vgmcont2.

The exponential conversion device 2C creates an output current whichchanges exponentially with respect to the differential output voltageVd3. The exponential conversion device 2D creates an output currentwhich changes exponentially with respect to the differential outputvoltage Vd4. These output currents are input to the current comparisoncircuit 3B. The current comparison circuit 3B changes the value of thegain control signal Vgmcont2 in accordance with the ratio of the outputcurrent of the exponential conversion device 2C and 2D.

Two reference input voltages Vref2 and Vref3 are input to the voltageconversion circuit 1C. The voltage conversion circuit 1C has acommon-mode detection circuit (CMD) 21, and a common-mode feedbackcircuit (CMF) 20. The gain control signal Vgmcont1 is input to thecommon-mode feedback circuit 20. The voltage conversion circuit 1Cconverts the reference input voltages Vref2 and Vref3 to differentialoutput voltages Vd5 and Vd6 on the basis of the ratio determined with again control signal Vgmcont3.

The current comparison circuit 4 creates the gain control signalVgmcont3 in accordance with a ratio between one of the differentialoutput voltages Vd5 and Vd6 (the differential output voltage Vd6 in thisembodiment) and the gain control signal Vgmcont2.

The reference input voltage Vref1 and the control input voltage Vc areinput to the voltage conversion circuit 1D. The voltage conversioncircuit 1D has a common-mode detection circuit (CMD) 21 and acommon-mode feedback circuit (CMF). The gain control signal Vgmcont1 isinput to the common-mode feedback circuit 20. The voltage conversioncircuit 1D converts the reference input voltage Vref1 and the controlinput voltage Vc to differential output voltages Vd7 and Vd8 on thebasis of the ratio determined with the gain control signal Vgmcont3.

Incidentally, one of the two differential output voltages (Vd8 in thisembodiment) becomes a gain. control signal Vgmont4 which determines thegain (ratio) of a voltage conversion circuit 1E.

The voltage conversion circuit 1E converts the reference input voltageVref1 and the control input voltage Vc into a differential outputvoltage on the basis of the ratio determined with the gain controlsignal Vgmcont4. An exponential conversion device 2E creates an outputcurrent Iout which changes exponentially with respect to thisdifferential output voltage.

Incidentally, the exponential conversion devices 2A, 2B, 2C, 2D and 2Emay comprise a field effect transistor (MOSFET) which is operated, forexample, in the weak inversion area. Furthermore, the exponentialconversion devices 2A, 2B, 2C, 2D, and 2E may comprise bipolartransistors instead of the electric field effect transistor.

FIG. 5 is a diagram showing an exponential conversion circuit accordingto a fourth embodiment of the present invention.

Each of a plurality of (n−1 in this embodiment, but n is a naturalnumber of 3 or more) master exponential conversion circuits 6-1, 6-2,6-3, . . . 6-(n−1) has a voltage conversion circuit 1, exponentialconversion devices 2A, 2B and a current comparison circuit 3.

The voltage conversion circuit 1A in a master exponential conversioncircuit 6-i (i is 1 through n−1) converts the two reference inputvoltages Vref1 and Vref2 to the first and the second differential outputvoltages on the basis of a gain control signal Vgmcont i. Theexponential conversion device 2A creates a first output current whichchanges exponentially with respect to the first differential outputvoltage while the exponential conversion device 2B creates the secondoutput current which changes exponentially with respect to the seconddifferential output voltage. Then the current comparison circuit 3creates the gain control signal Vgmcont i on the basis of the ratio ofthe first and second output currents.

The output signals Vgmcont1, Vgmcont2, Vgmcont3 . . . Vgmcont(n−1) of aplurality of master exponential conversion circuits 6-1, 6-2, 6-3, . . .6(n−1) are input to a polynomial circuit 7. Furthermore, a plurality (n)of reference input voltages Vref1, Vref2, Vref3 . . . Vrefn and thecontrol input voltage Vc are input to the polynomial circuit 7 and acontrol input voltage Vc is converted in accordance with a predeterminedfunction.

The slave exponential conversion circuit 8 has the voltage conversioncircuit 1B and the exponential conversion device 2C. The voltageconversion circuit 1B converts the reference voltage and the controlinput voltage Vc to a differential output voltage on the basis of theoutput voltage of the polynomial circuit 7. The exponential conversiondevice 2C creates an output current Iout which changes exponentiallywith respect to this differential output voltage.

Incidentally, the exponential conversion devices 2A, 2B, and 2C cancomprise an electric field effect transistor (MOSFET) which operates,for example, in the weak inversion area. Furthermore, the exponentialconversion circuit 2A, 2B and 2C can comprise bipolar transistorsinstead of the electric field transistor.

FIG. 6 is a diagram showing an exponential conversion circuit accordingto a fifth embodiment of the present invention. FIG. 7 is a diagramshowing embodiments of the voltage conversion circuits 1A and 1B of FIG.6.

In this exponential conversion circuit, the gain of the slave block(slave exponential conversion circuit) 303 is controlled with one masterblock (master exponential conversion circuit) 302.

Two reference input voltages X0 and X1 are input to the master block302, and the value of the gain control signal (output voltage) Z isdetermined on the basis of a difference in these reference inputvoltages X0 and X1.

In the voltage conversion circuit of FIGS. 6 and 7, an output current Ydisplayed in decibels of the block 303 becomes as follows:

Y=Y 0+G(Z)·(X−X 0)

This expression is converted as follows with the voltage conversioncircuit 1A and the exponential conversion circuits 2A and 2B in themaster block 302.

Y 1=Y 0+G(Z)·(X 1−X 0)

Y 2=Y 0+G(Z)·(X 1−X 0)

The current comparison circuit 3 in the master block 302 compares theoutput currents Y1 and Y2 output from the exponential conversion devices2A and 2B to output a gain control signal Z. Then, this gain controlsignal Z determines the gain (ratio) of the voltage conversion circuits1A and 1B.

In the master block 302, as a result of such a feedback control, thegain control signal Z and the ratio G(Z) is determined so that the ratioof the output currents Y1 and Y2 of the exponential conversion devices2A and 2C becomes a preset value.

For example, as shown in FIG. 8, in the case where the currentcomparison circuit 3 comprises a current mirror circuit in which thearea ratio of the MOS transistors M1 and M2 is set to 1:4 (“W” in W/Ldenotes a channel width while “L” denotes a channel length). The ratioof output currents I_(DM1) and I_(DM2) of the two exponential conversiondevices becomes 1:4.

Since Y1 and Y2 are represented in decibel display, an expression ofY1−Y2=2G(Z)·(X1−X0)=12 [dB] is provided.

Here, when X1−X0 is set to 0.5 [V], G(Z)=12 [dB] is provided per 1[V].Furthermore, since the gain (ratio) of the voltage conversion circuit 13in the slave block 303 is determined with the gain control signal Z, theinput and output relation in the slave block 303 will become as follows.

Y=Y 0+12·(X 1−X 0)

In this manner, in the exponential conversion circuit shown in FIGS. 6and 7, the current comparison circuit 3 in the master block 302determines the ratio of the output currents I_(DM1) and I_(DM2), so thatan output change portion per unit voltage (1V) is virtually affected bythe influence of the change in the environment conditions (temperature)and a disparity in the manufacturing process.

FIG. 9 is a diagram showing an exponential conversion circuit accordingto a sixth embodiment of the present invention.

The characteristic of the exponential conversion circuit of the presentinvention lies in that a derived function circuit 9 is provided whichhas a function of correcting a non-linearity of the slave block 303 inthe case where an output current Y in the decibel display of the slaveblock (slave exponential conversion circuit) 303 changes linearly withrespect to the control input voltage X.

In the case where the change ratio G of the master block 302 and theslave block 303 changes with the control input voltage X, an expressionof Y=Y0+G(X, Z)·(X−X0)is provided.

This non-linearity is a characteristic inconvenient for the exponentialconversion circuit. Therefore, in this embodiment, Z is controlled withZ so that G(X, Z) does not ostensibly depends on X. Consequently, Y isrepresented with the function which changes linearly with respect to X.

In the case where the non-linearity of G(X, Z) is weak, it is supposedthat Z is represented in the linear expression of X. An expression ofZ=Z0+a·(X−X0) is provided.

In this embodiment, since unknown variants are Z0 and a, two masterblocks 302 are necessary. In this case, an output current in the decibeldisplay in each of the master block becomes as follows:

Y 1=Y 0+G(X 1, Z 0)·(X 1−X 0)

Y 2=Y 0−G(X 1, Z 0)·(X 1−X 0)

Y 3=Y 0+G(X 2, Z 1)·(X 2−X 0)

 Y 4=Y 0−G(X 2, Z 1)·(X 2−X 0)

Two current comparison circuits 3A and 3B in the master block 302outputs gain control signals Z0 and Z1 having a value which increasesand decreases with the ratio of the input currents respectively. Thegain (ratio) of the current conversion circuits 1A and 1B is controlledwith these gain control signals Z0 and Z1.

In such feedback control, the values of Z0 and G(X0, Z0) are determinedso that the ratio of the output current of the exponential conversiondevices 2A and 2B becomes a preset value and the values of Z1 and G(X1,Z1) are determined so that the ratio of the output current of theexponential conversion devices 2C and 2D become a preset value.

For example,

Y 1−Y 2=2G(X 1, Z 0)·(X 1−X 0)=6 dB

Y 3−Y 4=2G(X 1, Z 0)·(X 2−X 0)=12 dB

are provided.

Here, suppose that X1−X0=0.5[V], X2−X0=1.0[V] are established,G(X0−Z0)=G(X2, Z1)=6 [dB] are provided. Then, the gain control signalsZ0 and Z1 are input to the derived function circuit (a linear polynomialcircuit) 9 as a reference voltage.

In this derived function circuit 9, the following analog signalprocessing is conducted. An output voltage UI when the reference inputvoltage X2−X1 to the master voltage conversion circuit 1C in the derivedfunction circuit 9 becomes as follows.

U 1=U 0+a·(X 2−X 1)

U0=Z0 is established with the common-mode feedback, and a is determinedso that U1=Z1 is established with the feedback of the differential mode.

a=(Z 1−Z 0)/(X 2−X 1)

At this time, the following expression is provided.

U=U 0+a·(X−X 1)

=Z 0+(Z 1−Z 0)·(X−X 1)/(X 2−X 1)

When U is used as a gain control signal Z of the slave block 303,Z=Z0+a·(X−X1)is provided.

When X=X1 is established, or when Z=Z0 or X=X2 is established, Z=Z1 isprovided. Since the gain (ratio) of the voltage conversion circuit 1E inthe slave block 303 is determined, G(X, Z) becomes a desired value,namely 6 [dB]/[V] at least in two points; X=X1 and X=X2.

In the case where the non-linearity of G(X, Z) is weak, 6 [dB]/[V] isprovided in the vicinity of X0 and X1. As compared with the exponentialconversion circuit of FIG. 6, a circuit error is small, and 6 [dB]/[V]can be secured over a wide scope.

FIG. 10 is a diagram showing an exponential conversion circuit accordinga seventh embodiment of the present invention.

In this embodiment, an output current Y in the decibel display of theslave block (slave exponential conversion circuit) has a non-linearityrelation with respect to the control input voltage X, and the followingresult is provided.

Y=Y 0+G(X, Z)·(X−X 0)

As a consequence, Z is controlled with X so that G(X, Z) ostensibly doesnot depend upon X (becomes definite with respect to X). As aconsequence, the output current Y in the decibel display comes to have alinear relation with respect to the control input voltage X.

In the exponential conversion circuit of FIG. 10, since it is assumedthat Z is expressed in a second-order function, the input and outputrelation is expressed in the following expression.

Z=Z 0+a·(X−X 1)+b·(X−X 1)²

In this embodiment, since three unknown variants Z0, a and b areprovided, three master logics 302 are required. In this case, the outputcurrents in the decibel display in each master block are as follows.

Y 1=Y 0+G(X 1, Z 0)·(X 1−X 0)

Y 2=Y 0−G(X 1, Z 0)·(X 1−X 0)

Y 3=Y 0+G(X 2, Z 1)·(X 2−X 0)

Y 4=Y 0−G(X 2, Z 1)·(X 2−X 0)

Y 5=Y 0+G(X 3, Z 2)·(X 3−X 0)

Y 6=Y 0−G(X 3, Z 2)·(X 3−X 0)

The three current comparison circuits 3A, 3B, and 3C in the master block302 outputs gain control signals Z0, Z1 and Z2 having a value whichincreases and decreases with a ratio of two input currents respectively.The gain (ratio) of the voltage conversion circuits 1A, 1B and 1C iscontrolled with this gain control signals Z0, Z1 and Z2.

In such feedback control, the values of Z0 and G(X1, Z0) are determinedso that the ratio of the output currents of the exponential conversiondevices 2A and 2B becomes a preset value, the values of Z1 and G(X2, Z1)are determined so that the ratio of the output currents of theexponential conversion devices 2C and 2D becomes a preset value, and thevalues of Z2 and G(X3, Z2) are determined so that the ratio of theoutput current of the exponential conversion devices 2E and 2F becomes apreset value

For example,

Y 1−Y 2=2G(X 1, Z 0)·(X 1−X 0)=6 db

Y 3−Y 4=2G(X 2, Z 1)·(X 2−X 0)=12 dB

Y 5−Y 6=2G(X 3, Z 2)·(X 1−X 0)=−6 dB

Here, suppose that X1−X0=0.5[V], X2−X0=1.0[V], X3−X0=−0.5[V], anexpression of G(X1, Z0)=G(X2, Z1)=G(X3, Z2)=6 [dB] is provided. Then,this gain control signals Z0, Z1 and Z2 are input to the second orderlinear polynomial circuit 7.

The second order polynomial circuit 7 includes two master voltageconversion circuits 1D and 1E, a derived function circuit (linearpolynomial circuit) 9 and a slave voltage conversion circuit 1H.

In the second order polynomial circuit 7, the following analog signalprocessing is conducted Output signals U1 and U2 at the time of givingthe reference input voltages X2−X1 and X3−X1 to the voltage conversioncircuits 1D and 1E in the polynomial circuit 7 become as follows.

U 1−U 0+a(K 0)·(X 2−X 1)

U 2=U 0+a(K 1)·(X 3−X 1)

a(K0), and a(1) are determined so that U0=Z is established in thecommon-mode feedback and U1=Z1, U2=Z2 are established in thedifferential mode feedback.

a(K 0)=(Z 1−Z 0)/(X 2−X 1)

a(K 1)=(Z 2−Z 0)/(X 3−X 1)

In the derived function circuit (linear polynomial circuit) 9, thefollowing analog processing is conducted The linear polynomial circuit 9comprises a master voltage conversion circuit 1F and a slave voltageconversion circuit 1G. When the reference voltage X3−X2 is given to themaster voltage conversion circuit 1F, the following expression is given.

K 1=K 0+a·(X 3−X 2)

In the common mode feedback, K0 is determined, and “a” is determined sothat K0 becomes K1 with the differential mode feedback.

 a=(K 1−K 0)/(X 3−X 2)

At this time, in the slave voltage conversion circuit 1G, the followingexpression is provided.

K=K 0+a·(X−X 2)

=K 0+(K 1−K 0)·{(X−X 2)/(X 3−X 2)}

An output signal of the linear polynomial circuit 9 is input to theslave voltage conversion circuit 1H in the second order polynomialcircuit 7. An output signal U of the slave voltage conversion circuit 1Hbecomes as follows.

U=U 0+a(K)·(X−X 1)

Here, suppose that a(K) is represented in the linear expression of K.

a(K 0)

=a(K 0)+{a(K 1)−a(K 0)}·(K−K 0)/(K 1−K 0)

=a(K 0)+{a(K 1)−a(K 0)}·(X−X 2)/(X 3−X 2)

When the common mode feedback control is conducted so that U0=Z0 isprovided, U=Z0 is provided when X=X1 is established. When X=X2 isestablished, a(K)=a(K0) is provided. As a consequence, the followingresult is provided.

U=U 0+a(K 0)·(X 2−X 1)

=Z 0+Z 1−Z 1=Z 1

Furthermore, when X=X3 is established, a(K)=a(K0) is established, thefollowing result is provided.

U=U 0+a(K 1)·(X 3−X 1)

=Z 0+Z 2−Z 0=Z 2

Thus, the target second order function is obtained.

Then, when an output signal U of this derived function circuit (linearpolynomial circuit) 9 is input to the slave voltage conversion circuit1H in the polynomial circuit (the second-order linear polynomialcircuit) 7 is used as a gain control signal Z of the slave voltageconversion circuit 1H in the polynomial circuit (second order polynomialcircuit) 7, a predetermined value, namely 6 [dB]/[V] is provided atleast in three points, X=X1, X=X2 and X=X3.

When the non-linearity of G(X, Z) is weak, a circuit error is small and6 [dB]/[V] is provided in the vicinity of three points (X0, X1, X2).furthermore, as compared with the exponential conversion circuit of FIG.6, a circuit error is small and 6 [dB]/[V] can be secured over a widescope.

Incidentally, in the above embodiment, the second order polynomialcircuit 7 is used. The present invention can be applied to thepolynomial circuit having a larger order number.

FIG. 11 is a diagram showing an exponential conversion circuit accordingto an eighth embodiment of the present invention.

The exponential conversion circuit of the present invention comprises areference block 301 for generating a reference voltage Vcom1, a masterblock 302 for determining the exponential conversion characteristic, anda slave block 303 for actually conducting the exponential conversioncharacteristic determined with the master block 302.

The reference block 301 comprises a MOS transistor M300. A source of theMOS transistor is connected to a ground point Vss, and the gate and thedrain are mutually connected. A current Iref flows through the MOStransistor M300, and a reference voltage (common mode reference voltage)is output from the drain thereof.

The master block (first exponential conversion circuit) 302 comprises avariable trans-conductance amplifier 305, a calculation amplifier(operation amplifier) 304, a current mirror circuits M304 and M305, MOStransistors M301 and M302, and resistor devices R301 and R302.

The calculation amplifier 304 receives reference voltages Vcom1 andVcom2 to output an output signal to the variable trans-conductanceamplifier 305. The reference voltage Vcom2 is created with the resistordevices R301 and R302. The reference input voltage Vref, a conductancecontrol signal vgmcont and an output signal of the calculation amplifier304 are input to the variable trans-conductance amplifier (Gm1) 305.

An output signal of the variable trans-conductance amplifier 305 isinput to the gate of the MOS transistors M301 and M302. A voltagebetween the gate and the source of the MOS transistor M301 is Vgs 301while a voltage between the gate and the source of the MOS transistorM302 is set to vgs302. Furthermore, when the current I_(DM1) which flowsthrough the MOS transistor M304 is set to m, the current I_(DM2) whichflows through the MOS transistor M304 becomes 1/m with the currentmirror circuit.

Then a control voltage (conductance control signal) Vslope whichdetermines the exponential conversion characteristic is output from theconnection point (drain) of the MOS transistors M302 and M305.

Here, on the master block 302, an input terminal of the common modereference voltage Vcom1, an input terminal of the reference inputvoltage Vref, an input terminal of the conductance control signalVgmcont and an output terminal of the control voltage Vslope fordetermining the exponential conversion characteristic are providedrespectively.

The slave block (second exponential conversion circuit) 303 comprises avariable trans-conductance amplifier 307, a calculation amplifier(operation amplifier) 306, a current mirror circuits M306 and M307, andresistor devices R303 and R304.

The calculation amplifier 306 receives reference voltages Vcom1 andVcom3, and outputs an output signal to the variable trans-conductanceamplifier 307. The reference voltage Vcom3 is created with the resistordevices R303 and R304. A control voltage (control input signal) Vc, aconductance control signal Vgmcont, and an output signal of thecalculation amplifier 306 are input to the variable trans-conductanceamplifier (Gm2) 307.

An output signal of the variable trans-conductance amplifier 307 isinput to the gate of the MOS transistor M303, A voltage between the gateand the source of the MOS transistor M303 is Vgs303. Furthermore, when acurrent I_(DM3) which flows through the MOS transistor M306 is set to n,the current ln(Icont) which flows through the MOS transistor M307becomes 1/n with the current mirror circuit.

Then, this exponential conversion output current (gain control signal)ln(Icont) is output from the drain of the MOS transistor M307.Incidentally, ln denotes a logarithm.

Incidentally, an input terminal of the common mode reference voltageVcom1, an input terminal of the control voltage (control input signal)Vc, a conductance control signal (control voltage which determines theexponential conversion characteristic) Vgmcont, and an output terminalof the exponential conversion output current (gain control signal)ln(Incont) are provided at the slave block 303, respectively.

By the way, in the exponential conversion circuit, limiting a change inthe gain with respect to the change in the control signal to a definitescope without being affected by the temperature change can realized bymaintaining the change ratio in the output signal ln(Icont) whichlinearly changes with respect to the control input signal Vc input tothe exponential conversion circuit.

Here, when this exponential conversion characteristic is grasped as alinear function of the exponential conversion output ln(Icont), theexponential conversion circuit realizes primarily the following twocircuit operations.

One operation is an operation which determines a piece of theexponential conversion characteristic while the other operation is anoperation which sets to a definite level an inclination of theexponential conversion characteristic. Incidentally, such operationcharacteristic is represented in the first quadrant of an operationcharacteristic diagram of the exponential conversion circuit shown inFIG. 11.

An operation of the exponential conversion circuit of FIG. 11 isspecifically conducted in the following manner.

In the beginning, when the reference input voltage Vref is input to themaster block 302, an output current of the variable trans-conductanceamplifier 305 flows through the resistor devices R301 and R302 so thatthe value of an output voltage Vo is determined. The output voltage Vodetermines the gate voltage of the MOS transistors M301 and M302.

At the same time, since the reference current Iref which flows throughthe reference block 301 creates a common mode reference voltage Vcom1,this common mode reference voltage Vcom1 is input to the calculationamplifier 304 in the master block 302 and the calculation amplifier 306in the slave block 303 respectively.

In the master block 302, an output voltage (control voltage) Vslope isdetermined on the basis of the output voltage Vo of the variabletrans-conductance amplifier 305. Furthermore, the output voltage Vslopeis fed back with the variable trans-conductance amplifier 305 as acontrol voltage (feedback signal) Vgmcont. The reference voltage(intermediate voltage) Vcom2 is determined with the output voltage Vo ofthe variable trans-conductance amplifier 305 and resistor devices R301and R302.

A circuit associated with the feedback loop in the master block 302realizes an operation of determining a cut piece ln(Iref) of FIG. 12.

Incidentally, the MOS transistor M300 in the reference block 301 isoperated in the weak inversion area, and the following relation isshown. $\begin{matrix}{I_{ref} = {k\frac{W}{L}\exp \frac{V_{com1}}{{nV}_{T}}}} & (3)\end{matrix}$

Incidentally, in the expression (3), symbol Iref denotes a drain currentof the MOS transistor M300. Symbol W denotes a channel width of the MOStransistor M300. Symbol L denotes a channel length of the MOS transistorM300. Symbol Vcom1 denotes an output voltage (common phase referencevoltage) of the reference block 301. Symbol V_(T) denotes a heatvoltage. Symbol n denotes a constant. Furthermore, symbol k has a valueassociated with the conductance n of the MOS transistor M300, and thevalue depends upon the manufacturing process of the integrated circuit.

In the master block 302, the MOS transistors M301 and M302 for receivingan output signal of the variable trans-conductance amplifier 305 areoperated in a weak inversion area, and the drain currents I_(DM1) andI_(DM2) which flows through the MOS transistors M301 and M302 arerepresented in the following Expressions (4) and (5). $\begin{matrix}{I_{DM1} = {K\frac{W}{L}\exp \frac{V_{gs301}}{{nV}_{T}}}} & (4) \\{I_{DM2} = {K\frac{W}{L}\exp \frac{V_{gs302}}{{nV}_{T}}}} & (5)\end{matrix}$

At this time, the values of Vgs301 and Vgs302 are represented in thefollowing expressions (6) and (7).

V_(gs301)=V_(com1) +G _(m1) ·R·V_(ref)  (6)

 V_(gs302)=V_(com1) −G _(m1) ·R·V_(ref)  (7)

Incidentally, in the expressions (6) and (7), symbol R denotesresistance values (representing resistance values same as R301 and R302)of resistor devices R301 and R302. Symbol Gm1 denotes a conductance ofthe variable trans-conductance amplifier 305. Symbol Vref denotes aninput voltage of the variable trans-conductance amplifier 305. SymbolsVgs 301 and Vgs 302 denote a voltage between the gate and the source ofthe MOS transistors M301 and M302 respectively.

Furthermore, with the current mirror circuit comprising MOS transistorsM304 and M305, a ratio of the drain current I_(DM1) of the MOStransistor M301 and the drain current I_(DM2) of the MOS transistor M302becomes m:1/m. When this is represented in a relation expression, thefollowing expressions are provided. $\begin{matrix}{\frac{I_{DM1}}{I_{DM2}} = {\frac{m}{1/m} = m^{2}}} & (8) \\{m^{2} = {{\frac{I_{DM1}}{I_{DM2}} - {\exp \quad \frac{V_{gs301} - V_{gs302}}{{nV}_{T}}}} = {\exp \quad \frac{2{{Gm1} \cdot R \cdot V_{ref}}}{{nV}_{T}}}}} & (9)\end{matrix}$

Here, when a logarithm of both sides are taken in the expression (9),the following relation expression is provided. $\begin{matrix}{{\ln \quad m^{2}} = \frac{2{{Gm1} \cdot R \cdot V_{ref}}}{{nV}_{T}}} & (10)\end{matrix}$

At this time, the exponential function current created by the MOStransistors M301 and M302 with respect to the reference input voltageVref is determined with a size ratio of the MOS transistors M304 andM305 constituting the current mirror circuit so that Vslope is outputfrom the master block 302.

This Vslope is input to the variable trans-conductance in the masterblock 302 again as Vgmcont (differential mode feedback). That is, thevalue of the conductance Gm1 of the variable conductance amplifier 305is controlled with Vslope (or Vgmcont).

As a consequence, an inclination of the exponential conversioncharacteristic is determined with respect to the reference input voltageVref. Furthermore, Vslope output from the master block 302 is also inputto the variable trans-conductance amplifier 307 in the slave block 303(differential mode feedback). That is, the value of the conductance Gm2of the variable trans-conductance amplifier 307 is also controlled withVslope (or Vgmcont).

In this manner, the control of the conductance Gm2 of the variabletrans-conductance amplifier 307 in the slave block 303 becomessubstantially the same as the control of the conductance Gm1 of thevariable conductance amplifier 305.

The conductances Gm1 and Gm2 are represented in a expression in thefollowing manner. $\begin{matrix}{{Gm1} = {{Gm2} = {\frac{{nV}_{T}}{{RV}_{ref}}\ln \quad m}}} & (11)\end{matrix}$

Incidentally, a series of the above operation is represented in thefourth quadrant in FIG. 12.

That is, both conductances Gm1 and Gm2 are determined with the masterblock 301. Then, when the control voltage Vc is input to the variabletrans-conductance amplifier 307, an output voltage Vc2 of the variabletrans-conductance amplifier 307 is determined. The MOS transistor M303receives Vc2, the MOS transistor M303 are operated in a weak inversionarea in the same manner as the MOS transistors M301 and M302.

Then, a drain current I_(DM3) of the MOS transistor M303 is representedin the following manner. $\begin{matrix}{I_{DM3} = {k\frac{W}{L}\exp \frac{V_{gs303}}{{nV}_{T}}}} & (12)\end{matrix}$

Here, Vgs3O3 can be modified in the following manner. $\begin{matrix}\begin{matrix}{V_{gs303} = {V_{com1} + {G_{m2} \cdot R \cdot V_{C}}}} \\{= {V_{com1} + {\left( {\frac{{nV}_{T}}{2{RV}_{ref}}\ln \quad m^{2}} \right){RV}_{C}}}} \\{= {V_{com1} + {\left( {\frac{{nV}_{T}}{2V_{ref}}\ln \quad m^{2}} \right)V_{C}}}}\end{matrix} & (13)\end{matrix}$

Incidentally, in the Expression (13), symbol R denotes resistance valuesof resistor devices R301 and R302 (it is supposed that both R301 andR302 have the same resistance values). Symbol Gm2 denotes a conductanceof the variable conductance amplifier 307. Symbol Vcom1 denotes areference voltage output from the reference block 301. Symbol Vgs303denotes a voltage between the gate and the source of the MOS transistorsM303, respectively.

Furthermore, a ratio of a drain current I_(DM3) of the MOS transistorM303 and an output current (exponential conversion characteristiccurrent) Icont of the slave block 303 becomes n:1/n with a currentmirror circuit comprising MOS transistors M306 and M307, Here, supposethat n is 1, Icont can be represented in the following manner.$\begin{matrix}\begin{matrix}{I_{comt} = {k\frac{W}{L}{\exp \left( \frac{V_{gs303}}{{nV}_{T}} \right)}}} \\{= {k\frac{W}{L}{{\exp \left( \frac{V_{com1}}{{nV}_{T}} \right)} \cdot \exp}\left\{ {\frac{\left( {\frac{{nV}_{T}}{2V_{ref}}\ln \quad m^{2}} \right)}{{nV}_{T}}V_{C}} \right\}}}\end{matrix} & (14)\end{matrix}$

At this time, the common mode reference voltage Vcom1 created with thereference block 301 is input to the calculation amplifier 304 in themaster block 302, and is also input to the calculation amplifier 306 inthe slave block 303. Symbol Vcom3 denotes a voltage (intermediatevoltage) at a connection point of the resistor devices R303 and R304determined in the common mode feedback.

Here, in the Expression (14), when the logarithm (ln) on both sides aretaken, the following relation expression can be obtained.$\begin{matrix}{{\ln \left( I_{cont} \right)} = {{\ln \left( I_{ref} \right)} + {\ln \left\{ {m\left( \frac{V_{c}}{V_{ref}} \right)} \right\}}}} & (15)\end{matrix}$

Incidentally, symbol m refers to m in a size ratio (m:1/m) of the MOStransistor in the master block 302.

From the above result, conductance Gm1 (=Gm2) is determined with themaster block 302. Furthermore, an inclination of the exponentialconversion characteristic is determined by using the conductance Gm2 inthe slave block 303. Furthermore, the common mode reference voltageVcom1 is input to both the master block 302 and the slave block 303 todetermine the piece of the exponential conversion characteristic.

Such an operation is represented in the third and the fourth quadrantsof FIG. 12.

That is, an inclination of the exponential conversion characteristicwhich determines the variable scope of the exponential conversioncircuit is determined with the size ratio (m:1/m) of the MOS transistorin the master block 302 and the reference input voltage Vref so that theinclination does not depend upon the characteristic change and thetemperature change of the active device.

Hereinafter, there will be explained an embodiment of the variable gainamplifier (variable gain amplifier) using the exponential conversioncircuit of FIG. 11.

FIG. 13 is a diagram showing a basic structure of a system using thevariable gain amplifier according to the present invention. FIG. 14 is adiagram showing a concrete example of the variable gain amplifier 10 ofFIG. 13.

As the gain control circuit 11, the exponential conversion circuit shownin FIG. 11 is used as it is. Furthermore, in this embodiment, thevariable gain amplifier 10 comprises MOS transistors M401, M402 andM403, resistor devices Ri4O1, Ri402, r401, r402 and a capacitor C.

In this system, in the beginning, the control voltage Vc is input to thegain control circuit (exponential conversion circuit) 11. Furthermore,the gain control signal ln, Vc (or ln(Icont)) is created with the gaincontrol circuit 11. On the other hand, in the variable gain controlcircuit 10, output signals Outp and Outm are created on the basis of theinput signals INp and INm.

Here, since the gain control signals ln and Vc (or ln(Incont)) outputfrom the gain control circuit 11 are input to the variable gainamplifier 10, the gain of the variable gain amplifier 10 changes on thebasis of the gain control signal lnVc (or ln(Icont)). That is, when thebias voltage Vbias which is a gate voltage of the MOS transistors M402and M403 is changed with the gain control signal lnVc (or ln(Icont)),the gain of the variable gain amplifier 10 can be freely changed.

FIG. 15 is a diagram showing a basic structure of a system using avariable gain amplifier according to the present invention.

As the gain control circuit 11, the exponential conversion circuit shownin FIG. 11 can be used as it is. In this embodiment, for the sake ofsimplification of the explanation, the variable gain amplifier 502 isoperated in a single input. The variable gain amplifier 502 comprisesMOS transistors M503 and M504, resistor devices Rin and RL and acapacitor C. Symbol Vin denotes an input signal of the variable gainamplifier 502, an output signal of the variable gain amplifier, and thegain of the variable gain amplifier 502 is controlled with the variablegain control signal Ibias (lnVc).

The reference block 301 comprises a bias circuit 501, and MOStransistors M507 and M300. Current Iref flows through the MOS transistorM300, and a drain voltage of the MOS transistor M300 is output as thereference voltage Vcom1.

The bias circuit 501 comprises MOS transistors M501, M502, M505 and M506and a resistor device Rs. A size ratio (drive force ratio) between theMOS transistor M501 and the MOS transistor M502 is set to 1:4. Here, inthe drawings, symbol W/L denotes a channel width/channel length.

By the way, in such a case, the size (drive force) of the MOS transistorM504 in the variable gain amplifier 502 is set to 4NW/L, the relationbetween the bias circuit 501 and the variable gain amplifier 502 isshown in the following expression (16). $\begin{matrix}{A_{v} = {2\frac{RL}{Rs} \times N}} & (16)\end{matrix}$

Here, symbol Rs denotes a resistance value of the resistor device is inthe bias circuit 501, symbol RL denotes a resistance value of theresistor device RL in the variable gain amplifier 502. Furthermore,symbol N denotes a value which is determined with a size ratio of theMOS transistor.

In this manner, when the gain control of the variable gain amplifier 502is conducted by using the exponential conversion circuit of the presentinvention, the variable scope of the gain of the variable gain amplifier502 and the gain characteristic thereof becomes hard to change withrespect to the characteristic change and the temperature change of theMOS transistor.

Incidentally, in the variable gain amplifier 502 of FIG. 15, in the casewhere the MOS transistors M501 and M502 in the bias circuit 501 and theMOS transistors M503 and M504 in the variable gain amplifier 502 areoperated in a strong inversion area, and the bias current Ibias of thevariable gain amplifier 502 is changed in the scope of one to ten timeswith the gain control circuit (exponential conversion circuit) 11, thevariable gain amplifier 502 can realize a gain change of 10 dB portion.

In contrast, in the variable gain amplifier 502 of FIG. 15, the MOStransistors M501 and M502 in the bias circuit 501 and the MOStransistors M503 and M504 in the variable gain amplifier 502 areoperated in a weak inversion area, respectively. Furthermore, in thecase where the bias current Ibias of the variable gain amplifier 502 ischanged by the gain control circuit (exponential conversion circuit) 11in the scope of one to ten times, the variable gain amplifier 502 canrealize a change in the gain of 20 dB portion.

Since the operation characteristic of the MOS transistor changes withthe bias voltage (weak inversion area ←→ strong inversion area), itbecomes difficult to realize a wide scope gain change with the gaincontrol circuit in the case where only one variable gain amplifier 502constitutes a system.

An embodiment will be explained to solve this problem.

FIG. 16 is a diagram showing a basic structure of a system using avariable gain amplifier according to the present invention.

The characteristic of this system lies in that a plurality of variablegain amplifiers are connected in series, and a wide scope of gain changecan be realized. As the gain control circuits 11A and 11B, theexponential conversion circuit shown in FIG. 11 is used as it is.

The gain of the former half of the plurality (two in this embodiment) ofvariable gain amplifiers VGA is controlled with the gain control signalIbias created with the gain control circuit 11A and the reference block301A. That is, in a portion surrounded by broken line 601, thesevariable gain amplifiers VGA are operated in a strong inversion areawith the gain control circuit 11A and the reference block (bias circuit)301A.

In contrast, the gain of the latter half of the plurality (three in thisembodiment) of variable gain amplifiers VGA is controlled with the gaincontrol circuit 11B and the gain control bias Ibias (exp.) created withthe reference block 301B. That is, in a portion surrounded with thebroken line 602, these variable gain amplifiers VGA's are operated in aweak inversion area with the gain control circuit 11B and the referenceblock (bias circuit) 301B.

As a consequence, in a system comprising the gain control circuit(exponential conversion circuit) and the variable gain amplifier shownin FIG. 16, it becomes possible to realize a gain change for 80 dBportion. For example, this system can be applied to a variable gainamplifier of an IF stage of the CDMA method. Furthermore, various gainamplifiers can be constituted by changing a combination of an amplifier.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the sprit or scope of the general inventiveconcept as defined by the appended clams and their equivalents.

What is claimed is:
 1. An exponential conversion circuit comprising: afirst voltage conversion circuit for converting first and secondreference input voltages to first and second differential outputvoltages on the basis of a first gain control signal; a firstexponential conversion device for creating a first output current whichchanges exponentially with respect to the first differential outputvoltage; a second exponential conversion device for creating a secondoutput current which changes exponentially with respect to the seconddifferential output voltage; a current comparison circuit for changingthe first gain control signal in accordance with a ratio of the firstand the second output currents; a second voltage conversion circuit forconverting a control input voltage and the first reference input voltageinto third and fourth differential output voltages, respectively, on thebasis of a first gain control signal; and a third exponential conversiondevice for creating a third output current which changes exponentiallywith respect to the third and the fourth differential output voltages.2. The exponential conversion circuit according to claim 1, wherein eachof the first and second voltage conversion circuits has a common-modedetection circuit and a common-mode feedback circuit.
 3. The exponentialconversion circuit according to claim 2, further comprising a logarithmconversion device so that a reference voltage is input to thecommon-mode feedback circuit, and that the reference voltage becomes alogarithm of the reference input current having a temperaturecharacteristic.
 4. The exponential conversion circuit according claim 1,wherein each of the first and second exponential conversion circuitscomprises an electric field effect transistor operated in a weakinversion area.
 5. The exponential conversion circuit according to claim1, wherein each of the first and second exponential conversion circuitcomprises a bipolar transistor.
 6. A variable gain circuit comprising:an exponential conversion circuit comprising: a first voltage conversioncircuit for converting first and second reference input voltages tofirst and second differential output voltages on the basis of a firstgain control signal; a first exponential conversion device for creatinga first output current which changes exponentially with respect to thefirst differential output voltage; a second exponential conversiondevice for creating a second output current which changes exponentiallywith respect to the second differential output voltage; a currentcomparison circuit for changing the first gain control signal inaccordance with a ratio of the first and the second output currents; asecond voltage conversion circuit for converting a control input voltageand the first reference input voltage into third and fourth differentialoutput voltages, respectively, on the basis of the first gain controlsignal; and a third exponential conversion device for creating a thirdoutput current which changes exponentially with respect to one of thethird and the fourth differential output voltages; and a plurality ofvariable gain amplifiers mutually connected in series wherein a gain ofeach of said plurality of variable gain amplifiers is controlled withthe third output current of the exponential conversion circuit.
 7. Avariable gain system comprising: a first variable gain circuit operatedin a strong inversion area; and a second variable gain operated in aweak inversion area connected in series to the first variable gaincircuit, the first variable gain circuit comprising: an exponentialconversion circuit comprising: a first voltage conversion circuit forconverting first and second reference input voltages to first and seconddifferential output voltages on the basis of a first gain controlsignal; a first exponential conversion device for creating a firstoutput current which changes exponentially with respect to the firstdifferential output voltage; a second exponential conversion device forcreating a second output current which changes exponentially withrespect to the second differential output voltage; a current comparisoncircuit for changing the first gain control signal in accordance with aratio of the first and the second output currents; a second voltageconversion circuit for converting a control input voltage and the firstreference input voltage into third and fourth differential outputvoltages, respectively, on the basis of the first gain control signal;and a third exponential conversion device for creating a third outputcurrent which changes exponentially with respect to one of the third andthe fourth differential output voltages; a first electric field effecttransistor wherein a gate and a drain are mutually connected, a sourceis connected to the ground point, and a second gain control signal ofthe exponential conversion circuit is provided to the gate as a biassignal; second and third exponential electric field effect transistorsconstituting a differential amplifier circuit, the second exponentialelectric field effect transistor outputting a first output signal on thebasis of a first input signal, the third electric field effecttransistor outputting a second output signal on the basis of the secondinput signal; a first resistor device connected between the gate of thefirst electric field effect transistor and the gate of the secondelectric field effect transistor; and a second resistor device connectedbetween the gate of the first electric field effect transistor and thegate of the third electric field effect transistor; wherein the firstelectric field effect transistor is operated in a strong inversion area,and a gain of the first variable gain circuit is controlled with thesecond gain control signal, the second variable gain circuit comprising:an exponential conversion circuit comprising: a first voltage conversioncircuit for converting first and second reference input voltages tofirst and second differential output voltages on the basis of a firstgain control signal; a first exponential conversion device for creatinga first output current which changes exponentially with respect to thefirst differential output voltage; a second exponential conversiondevice for creating a second output current which changes exponentiallywith respect to the second differential output voltage; a currentcomparison circuit for changing the first gain control signal inaccordance with a ratio of the first and the second output currents; asecond voltage conversion circuit for converting a control input voltageand the first reference input voltage into third and fourth differentialoutput voltages, respectively, on the basis of the first gain controlsignal; and a third exponential conversion device for creating a thirdoutput current which changes exponentially with respect to one of thethird and the fourth differential output voltages; a first electricfield effect transistor wherein a gate and a drain are mutuallyconnected, a source is connected to the ground point, and a second gaincontrol signal of the exponential conversion circuit is provided to thegate as a bias signal; second and third exponential electric fieldeffect transistors constituting a differential amplifier circuit, thesecond exponential electric field effect transistor outputting a firstoutput signal on the basis of a first input signal, the third electricfield effect transistor outputting a second output signal on the basisof the second input signal; a first resistor device connected betweenthe gate of the first electric field effect transistor and the gate ofthe second electric field effect transistor; and a second resistordevice connected between the gate of the first electric field effecttransistor and the gate of the third electric field effect transistor;wherein the first electric field effect transistor is operated in a weakinversion area, and a gain of the second variable gain circuit iscontrolled with the second gain control signal.
 8. A variable gaincircuit comprising: an exponential conversion circuit comprising: afirst voltage conversion circuit for converting first and secondreference input voltages to first and second differential outputvoltages on the basis of a first gain control signal; a firstexponential conversion device for creating a first output current whichchanges exponentially with respect to the first differential outputvoltage; a second exponential conversion device for creating a secondoutput current which changes exponentially with respect to the seconddifferential output voltage; a current comparison circuit for changingthe first gain control signal in accordance with a ratio of the firstand the second output currents; a second voltage conversion circuit forconverting a control input voltage and the first reference input voltageinto third and fourth differential output voltages, respectively, on thebasis of the first gain control signal; and a third exponentialconversion device for creating a third output current which changesexponentially with respect to one of the third and the fourthdifferential output voltages; a first electric field effect transistorwherein a gate and a drain are mutually connected, a source is connectedto the ground point, and a second gain control of the exponentialconversion circuit is provided to the gate as a bias signal; second andthird electric field effect transistors constituting a differentialamplifier circuit, the second electric field effect transistoroutputting a first output signal on the basis of a first input signal,the third electric field effect transistor outputting a second outputsignal on the basis of a second input signal; a first resistor deviceconnected between the gate of the first electric field effect transistorand the gate of the second electric field effect transistor; and asecond resistor device connected between the gate of the first electricfield effect transistor and the gate of the third electric field effecttransistor.
 9. A variable gain circuit comprising: an exponentialconversion circuit comprising: a first voltage conversion circuit forconverting first and second reference input voltages to first and seconddifferential output voltages on the basis of a first gain controlsignal; a first exponential conversion device for creating a firstoutput current which changes exponentially with respect to the firstdifferential output voltage; a second exponential conversion device forcreating a second output current which changes exponentially withrespect to the second differential output voltage; a current comparisoncircuit for changing the first gain control signal in accordance with aratio of the first and the second output currents; a second voltageconversion circuit for converting a control input voltage and the firstreference input voltage into third and fourth differential outputvoltages, respectively, on the basis of the first gain control signal;and a third exponential conversion device for creating a third outputcurrent which changes exponentially with respect to one of the third andthe fourth differential output voltages; a first electric field effecttransistor wherein a gate and a drain are mutually connected, a sourceis connected to a ground point, a second gain control signal of theexponential conversion circuit is provided to the gate as a bias signal;a second electric field effect transistor for outputting an outputsignal on the basis of an input signal; and a resistor device connectedbetween the gate of the first electric field effect transistor and thegate of the second electric field effect transistor.